1. Field of the Invention
The invention relates to a process and apparatus for asymmetric bladder compression for elimination of lamination defects on Progressive Addition Lenses (PAL).
The invention may be applied to any type of lamination process. Most preferred are transfer processes where one or more layers to be transferred are placed on a carrier, pressure being applied to the layers by means of the carrier instead of directly.
2. The Prior Art
Plastic lenses are becoming increasingly desirable because of their low weight and thin designs. Since plastic is softer than glass, plastic lenses are coated to impart the finished lenses with additional or improved optical or mechanical properties. Thus, it is usual practice to coat at least one surface of an ophthalmic lens, typically made of an organic material, successively, starting from the surface of the lens, with an impact resistant coating (impact resistant primer), a scratch resistant coating (hard coat), an anti-reflecting coating and, optionally, a hydrophobic top coat. Other coatings such as a polarized coating, photochromic or dye coatings may also be applied onto one or both surfaces of the ophthalmic lens.
Numerous processes and methods have been proposed for coating a face of an ophthalmic lens, as described generally in U.S. Pat. No. 6,562,466. As an efficient alternative to dip coating or spin coating, the coatings are first layered on to a thin carrier and then transferred to the lens surface. One such process is the transfer process, for example, as described in the patent application WO 2003/004255, one embodiment of it being the Back-Side Transfer (BST) for which the coating is applied on the back side of the lens (concave surface).
The Back-Side Transfer (BST) lamination process is used to quickly transfer a Hard Multi-Coat (HMC) layer from an HMC carrier to an ophthalmic lens. As showed in FIG. 1, an Elastomeric Membrane Fixture (Bladder) is used for this purpose. The inflatable membrane apparatus is more fully described in WO 2003/004255. As pressure is applied to one side of the elastomeric membrane, it deforms and contacts the HMC carrier. The carrier then deforms and contacts the lens In its final position, a combination of heat and pressure fix the HMC layer to the lens, enabling a transfer of the HMC layer from the carrier to the lens. The concave side of the lens can be either tonic, or spherical, or progressive in curvature. Further, the exact same process may be used to apply a HMC layer on a front side of a lens; the main differences are related to the inversion of the surface's curvature.
Progressive Addition Lenses (PAL) can be manufactured in both positive and negative powers with varying surface contours. They comprise a region designed to facilitate vision of distant objects and a near vision region wherein the power of the lens is adapted to facilitate vision of close-by objects by the wearer. Generally, the near vision zone is positioned at the bottom of the lens once it is mounted on a frame. The negative power PAL lenses have a surface depression at the near vision region. The carrier used for the transfer process does not achieve full pressure in the vicinity of the depression, resulting in a coating defect. Currently, there is no acceptable solution. Processing parameters, such as: 1) lamination pressure; 2) lamination height; 3) bladder size; and 4) bladder shape, have been tested. None of these can effectively reduce the defect. The carriers are delicate parts, typically having a thickness in the range of 1 mm or less. For example, carriers may be produced according to the method described in U.S. Pat. No. 7,854,865. As a result, if the lamination pressure is adjusted too high, there is a risk of damaging the carrier resulting in an improperly coated lens. The same issue can appear in back side transfer if the near vision zone is formed on the back side of the lens or in front side transfer if the near vision zone is partially formed by variation of the curvature of the front side of the lens.
As a result there is a need to adapt the bladder compression lamination method and apparatus to eliminate coating defects on PAL lenses.